1. Field of the Invention
The present invention relates to a liquid crystal display apparatus using a holographic optical element.
2. Description of the Related Art
There is a known color liquid crystal display (LCD) apparatus which irradiates light from a light source arranged in the back of a liquid crystal display (LCD) panel (LCD device). This LCD apparatus generally uses red (R), green (G) and blue (B) color filters corresponding to the pixels of the LCD panel. The light from the light source is colored at the time the light from the light source passes through the LCD panel. Using this phenomena, the LCD apparatus displays a color image. When the light from the light source passes through the LCD panel in this LCD apparatus, however, the color filters absorb lights of complementary color components. The LCD apparatus therefore has a poor efficiency of using light from the light source and suffers from too dark a display.
To overcome this problem, an LCD apparatus has been proposed which uses a holographic optical element to diffract the R, G and B wavelength light components of the light from the light source at different diffraction angles so that those wavelength light components are condensed on the respective pixels of the LCD panel corresponding to the R, G and B colors to thereby display a color image. In this LCD apparatus, the light source has a lamp located at the focus point of a reflector which has a parabolic surface. The reflector reflects the white light emitted from the lamp to become parallel light. The LCD panel comprises a liquid crystal (LC) cell, which has a liquid crystal sealed between a pair of transparent electrode substrates, an incident-side polarization plate, arranged in the side where the light from this LC cell enters, and an outgoing-side polarization plate arranged in the side from which the light from the LC cell goes out. The LC cell has pixels corresponding to R, G and B formed in a dot matrix form between the pair of transparent substrates, and a black matrix formed between the pixels.
The holographic optical element used in this LCD apparatus diffracts light of any wavelength with a single diffraction grating at a predetermined diffraction angle which varies in accordance with the wavelength. The holographic optical element is arranged in such a manner that the parallel light from the light source enters at a predetermined angle (e.g., about 60.degree.). The R, G and B wavelength light components of the light incident to the holographic optical element are diffracted at their respective diffraction angles which differ from one another, and are emitted toward the associated pixels of the LC cell. This LCD apparatus can therefore display color images without using color filters.
Because the condensation positions of the R, G and B wavelength light components, split by the holographic optical element, vary wavelength by wavelength, however, the R, G and B wavelength light components actually do not hit the associated pixels of the LC cell evenly.
This will be discussed more specifically. The incident angles of the R, G and B wavelength light components to the associated pixels of the LC cell differ from one another. The shape of the openings of the LC cell is not symmetrical with respect to each unit pixel consisting of three pixels of R, G and B due to the presence of the black matrix which covers TFTs (Thin Film Transistors). Strictly speaking, the light from the light source does not become parallel light. Because of those factors, the light of a certain wavelength may enter other pixels than its associated, predetermined pixel of the LC cell and the intensity of the light which hits each pixel of the LC cell significantly varies from one of the R, G and B wavelength light components to another.
The LCD apparatus therefore suffers a variation in the light intensities of the R, G and B wavelength light components which are to be displayed, disabling the accurate regeneration of white and reducing the display quality of color images.